Information concerning the size and velocity of particles in a dispersed multi-phase medium is essential for accurate modelling and design of certain industrially important equipment. Such equipment includes, for example, spray absorbers, bubble columns, pipeline reactors and other contacting equipment.
Laser Doppler techniques for single phase velocity measurements have been available for about 20 years. Such techniques utilize the frequency of the light scattered by micron sized particles passing through a measurement volume defined by the intersection of two coherent laser beams to determine particle velocity. More recently, systems have been developed to also extract particle size information from the light scattered signal using the visibility technique (Bachalo et al., "An Instrument for Spray Droplet Size and Velocity Measurements", Engineering for Power, Vol. 102, No. 4, October 1980), or the phase lag approach (Bachalo et al., "Phase/Doppler Spray Analyzer for Simultaneous Measurements of Drop Size and Velocity Distributions", Optical Engineering, Vol. 23, No. 5, September/October 1984).
In the visibility method of particle size measurement, the AC and Pedestal components of the Doppler signal generated by a particle passing through the crossed laser beams are separated. The ratio of the areas under the AC and Pedestal curves is proportional to particle size.
Crossed or referenced coherent laser techniques of the Bachalo et al. type depend upon the so-called Mie scattering phenomenon and, consequently, are most useful for particles below 150 microns in diameter. In response to the need for instruments which would measure larger particle sizes, an alternate technique was developed.
This alternative technique, first introduced by Ballik and Chan ("Fringe Image Technique for the Measurement of Flow Velocities", Applied Optics, Vol. 12, No. 11, November 1973) uses a single incoherent light beam passed through a Ronchi grating to generate the requisite fringes required for the Doppler technique. Semiat and Dukler ("Simultaneous Measurement of Size and Velocity of Bubbles or Drops: A New Optical Technique", AlChE Journal, Vol. 27, No. 1, January 1981) applied this Ronchi grating technique to the joint measurement of drop size/velocity distributions in dispersed phase processes. Semiat and Dukler use a transit time technique to measure drop size.
In the transit time size measurement technique, drops transversing a series of light and dark fringes generated by passing a laser beam through a Ronchi grating refract or reflect light which is collected by receiving optics. The frequency spectrum of the collected signal is used to measure particle velocity while the duration ("transit-time") of the decrease in beam intensity caused by blockage of the beam as particles pass through the fringes is used to measure particle size.
All of these prior art techniques are feasible in theory but suffer from certain practical disadvantages in practical implementation. A principal disadvantage results from the manner in which the grating images or cross laser beam patterns are imaged into the two-phase medium under measurement. In normal practice, the medium is enclosed in an opaque walled column or vessel. A viewing window is provided on a wall of the column or vessel and the crossed laser beams or fringe images are projected into the medium. In many applications, the depth of penetration is very limited due to the density of the dispersed medium. Thus, local measurements of particle size/velocity throughout a cross-section of the medium become difficult or impossible. Furthermore, it is difficult to maintain a stable focussed small volume fringe image within the medium from the relatively remote walls of the column or vessel. Furthermore, to project this image throughout different cross-sections of the medium necessitates extensive time consuming and laborious realignment of the optical projecting system.